The present invention relates to dental materials derived from gypsum and, more particularly, relates to dental stone or plaster compositions having improved hardness and strength properties when set.
Dental gypsum materials, identified chemically as calcium sulfate hemihydrate (CaSO.sub.4).sub.2.H.sub.2 O, are commercially obtained by calcining pulverulent natural gypsum, calcium sulfate dihydrate, CaSO.sub.4.2H.sub.2 O, either in air or under steam pressure at temperatures of 110.degree.-130.degree. C. If calcination is carried out in air, the hemihydrate product obtained is composed of long, needlelike crystals which are irregular in shape and comparatively porous in nature. This product is known generally as beta gypsum or dental plaster. If, however, the calcination is conducted under steam pressure in an autoclave, the resulting hemihydrate product is composed predominately of cleavage fragments and crystals in the form of rods or prisms. The autoclaved product is denser and less porous than the beta hemihydrate and is designated for convenience as alpha gypsum, or dental stone. There are, in turn, two types of commercial dental stone, as classified by the American Dental Association, Type III stone; and Type IV stone or dental stone, high strength. For purposes of convenience, these products may be referred to hereinafter as "buff" stone and "die" stone, respectively. The latter is the most highly refined grade of autoclaved gypsum and is used when positive molds or cast masters having optimum detail rendition are to be prepared. Buff stone, on the other hand, is a cheaper grade of autoclaved gypsum and yields products which are correspondingly inferior propertywise to those from the die stone.
Although gypsum-water mixtures such as the foregoing have heretofore provided hardened products with a number of desirable properties, these products oftentimes have exhibited inferior hardness and/or strength. This is because, in hardening, they have occluded a significant portion of the gauging water used in their preparation. By "gauging water" is meant that quantity of water in addition to the water of reaction which is needed to impart flowable consistency to a gypsum dispersion. In efforts to reduce the quantity of gauging water so as to produce stronger finished products, it has become common practice in the art to incorporate various modifying additives into the basic gypsum-water formulations. Noteable among such additives are various types of surfactants or superplasticizers. Specific surfactants so employed are a class of modified polycondensation products of formaldehyde and aryl or heterocyclic hydrocarbons, which products contain sulfonic acid groups. These are used typically in the form of neutral salts. For example, U.S. Pat. No. 2,941,890 (Zandberg et al.) describes a method for reducing the gauging water required in water dispersions of gypsum by incorporating therein a neutral sodium salt of a condensed aryl sulfonic acid as the sole dispersing agent or surfactant.
However, the use of such surfactants in gypsum dispersions has oftentimes proven unsatisfactory in consumer applications. While the overall water requirements have thus been reduced, the resulting dispersions have been too thin and difficult to handle. Foaming due to the presence of air bubbles has likewise been a problem.